This invention relates to an electronic digital thermostat control unit and its use in a Multipoint Temperature Controller for Refrigeration/Heating Systems and other systems such as in the automobile industry.
Electrical controls in refrigeration/heating systems basically comprise a simple thermostat, motor starting relay and an over load protector for controlling the motors. Larger models also incorporate a timer and a simple logic to control an electric heater (for the automatic defrost function). Some expensive models include one or more solenoids or motors to control blowers/air flow vanes for automatic temperature control in additional compartments of the unit.
The traditional apparatus for measuring and controlling temperatures in thermostats, consists of:
i. Gas/liquid filled capillaries in which the expansion/contraction of the gas/liquid with change in temperature is used to determine/control the temperature.
ii. Bi-metallic elements in which the deflection/deformation of a bi-metallic strip of two metals with widely different coefficients of thermal expansion, determines the temperature sensed by it.
iii. Mechanical bellows that are mechanically pushed by the expanding gas/liquid and that in turn move the mechanical contact and actuate the electrical circuit at a definite xe2x80x98setxe2x80x99 value.
iv. The deflecting bi-metallic strips itself performs the function of a moving mechanical switches that controls the electrical circuit.
These traditional methods/apparatus suffer from the following drawbacks:
a. Imprecise and imperfect sensing of temperatures
b. Low reliability
Analog thermostat units are also known in the art, see for instance the U.S. Pat. Nos. 3,666,973, 4,119,835, 4,137,770, 4,290,481, 4,638,850, 5,520,327, 5,528,017 and 5,592,989. However they suffer from the following drawbacks:
i) Tendency to drift with temperature and time
ii) Variation from unit to unit in behavior owing to the effect of tolerances in component values and characteristics
iii) Sensitivity to noise
Use of silicon diode for sensing temperature is also known, see for instance the U.S. Pat. No. 4,137,770 wherein a forward biased silicon diode is used in a bridge circuit for sensing the temperature. The analog thermostat described in this said US Patent is useable only for a fixed temperature and not for a variable temperature. Further, the use of the silicon diode for sensing temperature has a difficulty in its calibration over a temperature range. These limitations have not been addressed in the said US Patent.
Electronic digital thermostats are also available for use. These thermostats have been described, for instance, in U.S. Pat. Nos. 5,329,991; 5,107,918; 4,948,044; 4,799,176; 4,751,961; and 4,669,654. However, these electronic digital thermostats use expensive temperature sensors such as thermistors, thermocouples or platinum resistance thermometers. These sensors require complex and expensive interface circuits. This has made these thermostats unacceptable for use in all but the most expensive models of the refrigerators. In addition, many of the benefits of electronic thermostats, such as improved reliability of operation, are not effectively realized, when these are used with the conventional starting relay, over load protector, defrost timer, and the like. Replacing each of these elements with electronic equivalents or providing energy saving and other useful end-user functions, has so far proved to be economically viable in only the most expensive models of refrigeration units.
Conventional over load protection mechanisms are based on one of the following mechanisms:
a. Bi-metallic elements in which the deflection/deformation of a bi-metallic strip of two metals with widely different coefficients of thermal expansions determines the temperature sensed by it. The mechanical dimensions and profile of the bi-metallic strip determines the temperature at which the thermal trip action occurs to perform the over load protection function.
b. Positive temperature coefficient (PTC) resistance elements, the electrical resistance of which increases dramatically with increase in temperature beyond a certain xe2x80x98thresholdxe2x80x99 temperature, so that the resistance element effectively reduces the current in the electrical circuit to an insignificant value.
Both these methods have drawbacks. The bi-metallic over-load protector is a mechanically moving part that experiences electrical arcing every time it breaks the electrical circuit, causing electrical interference while at the same time resulting in corrosion of the contacts.
The PTC resistance element is similarly exposed to constant heat-cool cycles that create thermal stress and reduce reliability. At the same time, the electrical and temperature characteristics of the PTC element need to be matched with the load, in order to produce the correct electrical behavior. This limits flexibility and is at best a compromise in terms of effectiveness, as exact matching of PTC characteristics to the load characteristics is rarely possible.
Similarly, the conventional methods of implementing the starting relay functions and the associated problems are:
a. The use of a conventional mechanical relay, which suffers from the standard problem of electrical arcing and reduce reliability resulting from the use of a moving mechanical contact to make/break an electrical circuit.
b. The use of a positive temperature coefficient (PTC) resisted element which suffers from the same problems that are encountered in the use of a PTC element for the over load protector function.
The conventional defrost timer in a refrigeration system is an electro-mechanical or motorized timer mechanism. Since it has constantly moving mechanical parts, and an arcing electrical contact, its reliability is quite limited. Besides the problems listed above, conventional electrical controls in refrigeration systems have proved to be unwieldy and even expensive in terms of implementing multi-zone temperature control functions, that are desirable in larger refrigeration systems. In fact, some desirable functions that result in energy saving or provide useful features for the end user, are impractical to implement using such control mechanisms.
The object of this invention is to overcome the above mentioned drawbacks and provide an electronic digital thermostat which is cost effective, operationally safe and reliable.
A further object of this invention is to provide a single, multipoint compact electronic control unit by using the said electronic digital thermostat that overcomes all the above-mentioned drawbacks and provides the advantages of the expensive electronic controls currently available, at low cost.
To achieve the said objective this invention provides an electronic digital thermostat control unit which comprises:
a linear temperature sensing element,
a constant current source to drive the said linear temperature sensing element,
the output of said linear temperature sensing element is connected to an analogue-to-digital converter to produce a digital output,
the said digital output is connected to a circuit for correcting the sensitivity and offset values of the sensor using calibration data stored in non-volatile memory,
the corrected output is connected to one input of at least one digital comparator and the other input of each digital comparator receives a digital reference value from the said non-volatile memory or from variable control means,
the output of the said comparators is filtered using digital noise filters, to eliminate spurious outputs and is stored in a control latch to set/reset the input of a control latch whenever the output of digital comparator is xe2x80x98truexe2x80x99, for actuating the device in the consumer/industrial product that performs the temperature correction.
The said linear temperature-sensing element is resistance temperature detector and the said temperature detector is of platinum or nickel. The said liner temperature-sensing element employed in the present invention can sense temperature upto 650xc2x0 C.
The linear temperature sensing element might be a semiconductor chip for sensing temperature providing a linear current or voltage signal output or a linearized thermistor.
The output of the said control latch is connected to an output drive and protection circuit which monitors the load conditions continuously and deactivates the drive to the solid state switch, if overload conditions are encountered in the said consumer/industrial product. These overload conditions means thermal overload, over-current and turn-on inrush current conditions. Accordingly, the Output Drive and Protection Circuit includes a Thermal Protection circuit, an Over-current Protection Circuit and a xe2x80x98Soft Startxe2x80x99 circuit. The Thermal Protection Circuit monitors the temperature of the load, while the Over-current Protection Circuit monitors the current drawn by the load by determining the period for which the AC current signal exceeds a programmed DC reference value of the overload and the soft start circuit provides an effective reduced voltage start-up to the load during the initial period of the turn-on and thereby decreases the inrush current stress produced on the load in the case of motor and heater loads.
The temperature display unit is connected to one of the inputs of the said digital comparator(s) which receives its input from the output of the sensitivity and offset correction circuit and a selection switch permits the selective display of either the sensed temperature or the reference value from the digitized output from the potentiometer/switch.
A variable control means is provided in series with analogue-to-digital converter for varying the reference digital value fed to the digital comparator through a multiplexer for adjustment of the control limits of the temperature. The said variable control means is a potentiometer or switch which is connected to a switch debounce circuit and digital counter to remove spurious switch transitions and to increment/decrement a digital counter, the output of which is connected to the input of a digital multiplexer to determine whether the user control signal from the potentiometer/switch or the constant value from the non-volatile memory is to be used as a reference value for the digital comparator.
The output of the digital multiplexer is controlled by the signal from the selector switch through a switch debounce circuit.
The said digital comparator compares the corrected and sensed temperature with the reference value and generates a xe2x80x98truexe2x80x99/xe2x80x98falsexe2x80x99 output to set/reset a control latch after filtering through noise filters to eliminate spurious outputs.
One of the digital comparators receives a fixed reference value from the non-volatile memory and the other digital comparator receives its reference value either from the non-volatile memory or from a user variable control depending upon the state of a selector switch that toggles the selection
The power supply used for powering the electronic digital thermostat control unit preferably consists of a low loss capacitive voltage dropping network followed by a voltage clamping device, a rectifier and a filter network to provide a D.C. voltage. The said D.C. power supply provides an output in the 3-6 volts range.
A clock oscillator is connected to each circuit of the electronic digital thermostat control unit for providing the timing signals for the operation of each circuit. The said clock oscillator is a quartz clock oscillator operating in the 4-8 MHz frequency range.
The entire control circuit is implemented as a custom Application Specific Integrated Circuit (ASIC) to provide a miniature and cost effective thermostat excluding the sensor, a variable user control potentiometer/switch, the selector switch, temperature display unit and the solid state switch.
In other embodiments the said ASIC excludes non-volatile memory, clock circuit and the power supply in order to provide larger non-volatile memory capacities for storage of temperature data and interfaces to different types and sizes of displays in one embodiment, and in other embodiment further excludes the output drive and protection circuit in order to facilitate the use of higher power solid state switch, or to provide flexibility of control in multipoint applications.
To achieve the second objective this invention provides an electronic multi-point temperature control unit comprising:
a plurality of electronic thermostat control units as described above having a common non-volatile memory that stores reference and calibration data, for controlling the temperature in the required number of places in the refrigeration/heating systems, wherein
the outputs from the control latch units of the said electronic thermostat units are connected to logic circuit which selectively connects the outputs to one or more Output Drive and Protection Circuits using the data stored separately in the non-volatile memory of the electronic thermostat unit, the said Output Drive and Protection circuits drive and monitor the load, through solid state switches,
a central control unit connected to:
i. each of the said outputs from the control latch units of electronic thermostat control units and the inputs of the said output drive and protection circuits for enabling or disabling the said electronic thermostat control units and output of said drive and protection circuits depending upon the combination of the output from the electronic thermostat control unit and the user control input received from the potentiometer or digital counters, as well as the occurrence of fault conditions.
ii. a system timers unit which generates the timing signals for enabling/disabling one or more of the said Output Drive and protection circuits during special modes of operation,
iii. a starting relay circuit which provides the signals required to control one or more output drive and protection circuits at the time when the said load is to be switched on.
Any one or more of the Output Drive and Protection Circuit includes a thermal protection circuit, an over-current protection circuit and a xe2x80x98Soft Startxe2x80x99 circuit, the thermal protection circuit monitors the temperature of the load, while the over-current protection circuit monitors the current drawn by the load by determining the period for which the AC current signal exceeds a programmed reference value of the overload and the xe2x80x98Soft Startxe2x80x99 circuit provides an effective reduced voltage start-up to the load, during the initial period of the turn-on, and thereby decreases the in-rush current stress produced on the load in case of motor and heater loads.
The said Central Control Unit is a Logic circuit for implementing special functions e.g. Automatic Defrost and Quick-Freeze in the case of Refrigerators, and Timed Heating Cycles in case of Heating Systems. The central control unit and the non-volatile memory are programmed to control the functions of each component of electronic thermostat control unit and multipoint temperature controller dependent upon the requirements of the customers.
A display unit is connected to the output of one of the said electronic thermostat control units for displaying temperature.
At least one switch is connected through a switch debounce circuit and a digital counter to the input of the said central control unit for providing the user control signal required to operate the said electronic multi-point temperature control unit.
The entire control circuit is implemented as a custom Application Specific Integrated Circuit (ASIC), to provide a miniature and cost effective electronic multipoint temperature control unit, excluding the sensors of the electronic thermostat control units, variable user control switch(es), selector switch, temperature display unit, power supply and solid state switches.
In another implementation, the said ASIC excludes non-volatile memory, clock circuit and the power supply in order to provide larger non-volatile memory capacities for storage of temperature data and interfaces to different types and sizes of displays.